1. Technical Field
The present invention is related to an initial position determination method and the like that determines an initial position when positioning calculation is executed based on satellite signals from positioning satellites.
2. Related Art
A global positioning system (GPS) that uses a positioning signal is widely known, and is used as a positioning apparatus incorporated in, for example, a portable phone or a vehicle navigation system. In a GPS, the positioning calculation is done by obtaining values of four parameters, namely three-dimensional coordinate values representing the location of the system itself and a timepiece error, based on the information such as the locations of a plurality of GPS satellites or pseudo-ranges between the GPS satellites and the system itself. Thus, the position of the present location of the system itself can be known.
In the positioning by the GPS, since the position (hereinafter referred to as an “initial position”) of the positioning apparatus, which is used as an initial value of the positioning calculation (more accurately, position converging calculation), is required, various technologies for obtaining the initial position have been invented. For example, JP-A-2006-71460 discloses a technology for determining the initial position used in present positioning calculation by using the positioning error included in the positioning location obtained by the previous positioning calculation.
The positioning apparatus in the related art generally determines the initial position using a fractional part (code phase) of the pseudo random noise (PRN) code as a kind of spread code carried by the satellite signal from the GPS satellite. The PRN code is a signal with a bit rate of 1.023 Mbps and a bit length of 1,023 bits (=1 msec=300 km). Therefore, if the initial position is in an error range of within 150 km from the true position, the position converging calculation becomes possible using the fractional part (the code phase) of the PRN code. Therefore, in the positioning calculation in the related art, the error of the initial position is required to be within 150 km.
However, in reality, in some cases, it is difficult to obtain the initial position close to the true position of the positioning device. The case in which the positioning apparatus is powered on to first perform positioning after moving by air is a typical example. Depending on circumstances, in this case, it is also possible in which the initial position is separated by 500 km or even 1,000 km distance from the true position of the positioning apparatus. Thus, there is a need for a GPS can determine an initial position close to the true position of the positioning apparatus even in the aforementioned type of case. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.